Detection system for optical beam pointing and shaping

ABSTRACT

Apparatus for circularizing an optical beam and directing the beam into an optical system has mutually perpendicular X, Y, and Z axes. The Z-axis is the propagation axis of the beam. A detector arrangement receives a sample of the optical beam. The detector arrangement has four detector elements arranged in four quadrants defined by an x-axis and a y-axis of the detector arrangement, perpendicular to each other. The x and y-axes of the quadrant detector are arranged at 45 degrees to the X and Y-axes of the apparatus. Signals from the four detector elements provide information on the extent to which the beam is not circular and on any displacement of the beam in the X and Y-axes.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates generally to the use of quadrant detectors for determining position of a projected optical beam. The invention relates in particular to a method of determining the position and shape of a projected optical beam from information derived from a single quadrant detector.

DISCUSSION OF BACKGROUND ART

[0002] A four-quadrant detector is commonly used for determining position of a projected optical beam in an optical apparatus, and providing signals that are used to drive one or more movable optical components for steering the beam. One example of a laser beam to be steered is an output beam from a laser.

[0003] A laser output beam may be subject to variations in its direction and shape as a result of changes in parameters of the output beam such as output power, pulse width, pulse repetition frequency, and the like. The shape of a laser beam can be modified by passing it through an optical system including one or more optical elements that have different optical power in different transverse optical axes. Variations in the shape of the output beam can be corrected by moving one or more elements of the optical system with respect to another to change the magnification of the system in one of the axes in preference to the other. It would be advantageous to derive information regarding the position and shape of a laser output beam from a single quadrant detector.

SUMMARY OF THE INVENTION

[0004] The present invention is directed to apparatus for circularizing an optical beam and directing the beam into an optical system. The apparatus has mutually perpendicular X, Y, and Z axes, the Z-axis being the propagation axis. In one aspect of the invention, the apparatus comprises a detector arrangement arranged to receive a sample of the optical beam. The detector arrangement has first, second, third, and fourth detector elements arranged in first, second, third, and fourth quadrants defined by an x-axis and a y-axis of the detector arrangement, perpendicular to each other. The x and y-axes of the quadrant detector are arranged at 45 degrees to the X and Y-axes of the apparatus.

[0005] The 45-degree arrangement of the quadrant axes with respect to the optical system axes provides that signals from all four detector elements can be used to determine the extent to which the beam is not circular. Signals from opposite ones of the detector elements can also be used to determine the extent to which the beam is displaced from the Z-axis in the X and Y-axes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.

[0007]FIG. 1 schematically illustrates an optical system including a laser delivering an output beam, an axially preferential beam-expander including a movable element for changing the cross-section shape of the beam, a bi-axially tiltable mirror for changing the direction of the beam, and a quadrant detector and associated circuitry arranged in accordance with the present invention for deriving signals representative of the shape and position of the laser output beam and moving the movable optical element and tilting the tiltable mirrors in accordance with those signals.

[0008]FIG. 2 is an isometric view schematically illustrating the arrangement of optical elements of the beam expander of FIG. 1 with respect to Cartesian transverse axes of the optical system of FIG. 1.

[0009]FIG. 3 schematically illustrates the arrangement of Cartesian quadrant axes of the quadrant detector of FIG. 1 with respect to the Cartesian transverse axes of the optical system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0010] Turning now to the drawings, FIG. 1 schematically illustrates apparatus 20 in accordance with the present invention. The apparatus is used for shaping a laser beam 22 received from a laser 24 and delivering a shaped beam 26 to an optical system (not explicitly shown).

[0011] Continuing with reference to FIG. 1, and with reference additionally to FIG. 2, apparatus 20 includes a single-axis, afocal beam-expander 30. Beam expander 30 includes a cylindrical negative optical element 32 and a cylindrical positive element 34. The optical elements are aligned in a Cartesian coordinate system having an Z-axis defining a longitudinal axis or axis of propagation, and X-axis and a Y-axis perpendicular to each other and perpendicular to the Z-axis. The X, Y, and Z-axes are here considered as the axes of beam expander 30 and of the entire optical system of apparatus 20. Optical elements 32 and 34 have zero optical power in the Y-axis. Element 32 has negative optical power in the X-axis and element 34 has positive optical power in the X-axis. Element 34 is movable by an actuator 36 (see FIG. 1) along the Z-axis as indicated by double arrow M.

[0012] Beam 22 on leaving laser 24 has an elliptical cross-section shape, indicated in FIG. 2 by ellipse 38, having its major axis aligned in the Y-axis of beam-expander 30. Beam 22 is collimated in both the X-axis and the Y-axis. Beam expander 30 increases the beam dimension in the X-axis while the beam dimension in the Y-axis is essentially unchanged. The power of the optical elements and the position of element 34 with respect to element 32 is selected such that transmitted (shaped) beam 26 ideally has a circular cross section, indicated in FIG. 2 by circle 40. The beam expander is arranged to be nominally an afocal beam expander with shaped beam 26 leaving element 34 collimated in both the X-axis and in the Y-axis. Variations in elliptical cross-section 38 due to changes in operating parameters of laser 24 can be compensated by moving optical element 34 of beam expander 30 as indicated by double arrow M. Variations in the propagation direction of beam 22, and accordingly of transmitted beam 26, can be at least partially compensated by reflecting beam 26 sequentially off mirrors 42 and 44. Mirror 42 is tiltable, by actuators 43X and 43Y, in the X-axis and the Y-axis respectively as indicated in FIG. 1 by double arrows T_(X) and T_(Y) respectively. Mirror 44 is tiltable, by actuators 45X and 45Y, in the X-axis and the Y-axis respectively as indicated again in FIG. 1 by double arrows T_(X) and T_(Y) respectively. A preferred arrangement in accordance with the present invention for detecting changes in shape or direction of the shaped beam is described below with continuing reference to FIG. 1 and with reference additionally to FIG. 3.

[0013] Following reflection from mirror 42 and 44, beam 26 is transmitted through a beamsplitter 46, which reflects a sample 26S, for example about one percent, of beam 26 onto a quadrant detector arrangement 48. Referring to FIG. 3, detector arrangement 48 (here viewed from the side thereof on which sample 26S is incident) each include four detector elements 51, 52, 53, and 54 arranged in quadrants of a Cartesian axis system having an x-axis and a y-axis perpendicular to each other. The x-axis and the y-axis are arranged at forty-five degrees (45°) to respectively the X-axis and the Y-axis of beam expander 30, and, correspondingly, at 45° to the X and Y-axes of apparatus 20. The x-axis and the y-axis of the detector arrangement intersect at the intersection of the X-axis and the Y-axis. Dotted circles 56, 58, and 60 schematically respectively indicate the shape of an under-corrected elliptically shaped beam, an over-corrected elliptically shaped beam, and a corrected (circular shaped) beam. All are depicted as centered on the intersection of the quadrant x-axis and the y-axis, indicating, in this case, that the beam 26 from which they are sampled, here, is centered on the Z-axis of beam expander 30.

[0014] Quadrant detector elements 51, 52, 53, and 54, provide signals A, B, C, and D respectively. The magnitude of each signal depends on the portion of a beam that is incident on the detector element providing that signal. The inventive 45° inclination of the quadrant x-axis and y-axis with the optical X-axis and Y-axis of beam expander 30, allows signals A, B, C, and D to be processed by a controller 68 to provide signals representative of the departure from a circular shape of an incident beam sample 26S, and also to provide a signal representative of the displacement of the beam sample from the intersection (origin) of the x-axis and the y-axis, and correspondingly the displacement of beam 26 from the Z-axis at detector 48.

[0015] In most cases, changes in direction of the shaped beam and changes in beam cross-section shape as a result in changes in output parameters of the beam will be relatively small. By way of example, beam direction changes may be on the order of a few microradians. Further by way of example, cross-sectional shape changes from ideally circular may result in an elliptical cross section having an ellipticity (ratio of major axis to minor axis) of about 1.1.

[0016] If beam 26 is symmetrical about the X and Y axes, in cases where the change in beam direction is relatively small, the displacement of the beam from the Z-axis at detector 48 will also be relatively small, in which case, the X and Y-axis displacement of the beam, independent of the shape of the beam, can be represented by quantities:

X=(B−C)/(B+C)   (1)

and

Y=(D−A)/(D+A)   (2)

[0017] where X and Y represent coordinates in the X and Y-axis system, and, when X and Y are zero, beam 26, independent of the shape thereof, is centered on the Z axis. X and Y can amplified, and further processed if necessary, to provide command signals S1 _(X) and S1 _(Y) respectively (see FIG. 1) for driving mirror 42 via actuators 43X and 43Y to at least partially compensate for the change in beam direction.

[0018] Ellipticity of the beam, i.e., the extent to which the beam is not circular, can be determined by a quantity:

E=[(A+D)−(B+C)]/(A+B+C+D)   (3)

[0019] when beam 26 is centered on detector 48. Accordingly, when X, Y, and E are zero, beam 26 has a circular cross-section shape. Quantity E can be amplified and further processed if necessary to provide a signal S2. Signal S2 commands actuator 36 to move element 34 until the value of E is zero, at which point beam 26 has a circular cross section at the detector, i.e., at a point along the Z-axis represented by sample 26S on detector 48. It should be noted, here, that equation (3) is valid only if all of sample 26S is collected by detector 48.

[0020] A consequence of moving element 34 of beam expander 30 to maintain a circular shape of beam 26 at detector 48 is that the afocal arrangement of the beam expander will be disturbed, possibly resulting in some difference in divergence of the beam in the X-and Y-axes, i.e., astigmatism. If any astigmatism is present in beam 26, this would manifest itself in that a sample beam having a circular cross-section at a point along the Z-axis represented by sample 26S on detector 48 would not have a circular cross-section at detector arrangement at some other point on the Z-axis. Such astigmatism could be detected by a second sampling beamsplitter and detector arrangement in accordance with the present invention located along the Z-axis in the direction of propagation.

[0021] Such a beamsplitter is depicted in FIG. 1 as beamsplitter 49. Beamsplitter 49 directs a sample 26F of the beam 26 onto a detector arrangement 48A configured similar to detector 48 of FIG. 3, with the quadrant axes of the detector having their origin on the Z-axis. Detector 49 provides signals A1, B1, C1, and D1 corresponding to signals A, B, C, and D of detector 48. A signal representative of the cross-section shape of the beam at the second detector can be determined as described above with reference to detector 48 and be used to monitor astigmatism or operate a device (not shown) for correcting the astigmatism. By way of example such an astigmatism-correcting device could be a tiltable plate or one or more additional, axially movable optical elements.

[0022] Signals A1, B1, C1, and D1 also provide X and Y axis displacement information and are used to generate command signals S3 _(X) and S3 _(Y) for operating mirror 44. Mirrors 42 can be operated cooperatively such that sample 26S and 26F are centered on detectors 48 and 48A, thereby fully correcting any direction change of beam 26 leaving laser 24.

[0023] Those skilled in the art will recognize that other variations of the above-described beam pointing and shaping apparatus are possible without departing from the spirit and scope of the present invention. By way of example, beam expander 30 may include more than two optical elements, with more than one element movable with respect to another, and with one or more elements being spherical elements. Any “cylindrical” element may have finite optical power in both the X-axis and the Y-axis but with optical power in one of the axes being greater than that in the other. Regarding the inventive 45° degree inclination of the quadrant detector x-axis and y-axis with respect to the X-axis and Y-axis of beam expander 30, those skilled in the art may determine other signal processing algorithms for providing errors signals representative of beam-shape, astigmatism, or beam-displacement, without departing from the spirit and scope of the present invention.

[0024] In summary, the present invention is described above with respect to a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto. 

What is claimed is:
 1. Apparatus for circularizing an optical beam and directing the beam into an optical system, the apparatus having mutually perpendicular X, Y, and Z axes, the Z-axis being the propagation axis, the apparatus comprising: a first detector arranged to receive a first sample of the optical beam, said detector having first, second, third, and fourth detector elements arranged in first, second, third, and fourth quadrants defined by an x-axis and a y-axis of said detector arrangement, perpendicular to each other, said quadrants being numbered in sequential rotational order around said x and y-axes; and wherein, said x and y-axes of said detector arrangement are arranged at 45 degrees to said X and Y-axes of the apparatus.
 2. The apparatus of claim 1, wherein said first, second, third, and fourth detector elements are arranged to provide respectively first, second, third, and fourth electrical signals on receiving said optical-beam sample, said first, second, third, and fourth electrical signals being electronically processed to provide a first error signal representative of a degree to which the cross-section shape of said optical beam is not circular on said first detector.
 3. The apparatus of claim 2, wherein said first and third electrical signals and said second and fourth electrical signals are electronically processed to provide respectively second and third error signals representative of magnitude of any displacement of the optical beam from the Z axis, on said first detector, in respectively the X and Y-axes.
 4. The apparatus of claim 3, further including a first optical device for changing the direction of said optical beam.
 5. The apparatus of claim 4, wherein said first optical device is a tiltable mirror.
 6. The apparatus of claim 5, wherein said tilting of said mirror is controlled by a second and third command signals representative of said second and third error signals.
 7. The apparatus of claim 2, further including a lens including one optical element movable for changing the cross-section shape of the beam.
 8. The apparatus of claim 7, wherein said moving of said optical element is controlled by a first command signal representative of said first error signal.
 9. The apparatus of claim 1, further including a second detector arranged to receive a further sample of the optical beam, said second detector arrangement having first, second, third, and fourth detector elements arranged in first, second, third, and fourth quadrants defined by an x-axis and a y-axis of said detector arrangement, perpendicular to each other, and wherein, said x and y-axes of said second detector arrangement are arranged at 45 degrees to said X and Y-axes of the apparatus.
 10. The apparatus of claim 9, wherein said first, second, third, and fourth detector elements are arranged to provide respectively fifth, sixth, seventh, and eighth electrical signals on receiving said optical-beam sample, said fifth, sixth, seventh, and eighth electrical signals being electronically processed to provide a fourth error signal representative of a degree to which the cross-section shape of said optical beam is not circular on said second detector.
 11. The apparatus of claim 9, wherein said fifth and seventh electrical signals and said sixth and eighth electrical signals are electronically processed to provide respectively fourth and fifth error signals representative of magnitude of any displacement of the optical beam from the Z axis, on said second detector, in respectively the X and Y-axes.
 12. The apparatus of claim 11, further including a second optical device for changing the direction of said optical beam.
 13. The apparatus of claim 12, wherein said second optical device is a tiltable mirror.
 14. The apparatus of claim 13, wherein said tilting of said mirror is controlled by a fourth and fifth command signals representative of said fourth and fifth error signals.
 15. A detector arrangement for monitoring the position and shape of a propagating laser beam comprising: a quadrant detector positioned in the path of the beam, with each quadrant generating an independent output signal; and a processor for monitoring the output signals, said processor determining the lateral position of the beam by comparing the differences in output signals between opposite quadrants and for determining the circularity of the beam by comparing the difference between sum of the signals from a first pair of opposing quadrants and the sum of the signals from the remaining pair of opposing quadrants.
 16. A detector arrangement for monitoring the position and shape of a propagating laser beam comprising: a quadrant detector positioned in the path of the beam, with each quadrant generating an independent output signal; and a processor for monitoring the output signals, said processor determining the lateral position of the beam along one axis by comparing the difference in output signals between a first pair of opposite quadrants and along a second perpendicular axis by comparing the difference in output signals between the remaining pair of opposite quadrants, said processor further determining the circularity of the beam by comparing the difference between sum of the signals from the first pair of opposing quadrants and the sum of the signals from the remaining pair of opposing quadrants. 